Guidelines to develop a Control System:

In Figure 29 is shown the schematic in the expansion module that helps to acquire the analog signal. When the process is made, the signal passes through a series of resistances, then through an instrumental amplifier (OPAM) that ensures to adjust the signal to the enclosed values, then through a gain adjust and finally through a ADC with the sample and hold (inside it) to retain the value of the signal for the time in which the conversion to a 12-bit word is made.

Figure 29: Schematic for the acquisition of a signal of the expansion module SIMATIC EM 231 [9].

Later the acquired data is saved in an excel file that has to be analyzed and processed with statistical model thorough the MATLAB Software, to create and produce a model that can be compared with the real results of the project in the DIMEAS laboratory. This is section is called statistical models; a further section will emphasize in this matter. For further specifications in the Analog Module EM 231, please refer to the annexes.

Now that the control device and the actuators have been described, there will be a short introduction to the recommendations that must be followed to produce an efficient and productive control system. To develop a control system with a PLC there are many methods to address the problem and design a suitable solution. Nevertheless, this process is very important and there are some basic and general guidelines that can be helpful to develop any program and the ones that were used in this thesis project to design the control of the PLC.

The following steps can be used to design an efficient code to control any project and a variety of methodologies are based in this few steps.

1. Divide the process: Having the whole process or machine is difficult to determine which necessities are important and is easy to have a misconception of what is really needed.

Dividing the process into sections that are independent from each other helps to emphasize what is needed to solve each particular portion and to made a proper decision to ensure an efficient solution exploiting resources as best as possible [15].

2. Create Functional Specifications: Describe operations made by each of the sections, recognize the proper input and outputs that are pertinent for the functional description of the operation and states that must be achieve from one action to another one. Identify the actuation methods that are implemented; solenoids, motors, drives, etc. Describe the interaction of the machine and the user, the human machine interface (Manual Pad) and the possible interactions of the operator and other sections of the machine [15].

3. Design the Safety Circuits: First of all, the equipment required during the hard wired and logic safety must be identified. The control devices implemented can collapse in an unusually dangerous manner, occasioning unexpected startup or change in the operation of the machine.

Unexpected and improper operation can result in dangerous physical injury to people or significant damage of the property. The following considerations are given to be aware when electro-mechanical devices, in addition to the PLC are used to prevent unsafe operations [15]:

a. Identification of improper and unexpected operation actuators that can be

b. Identify the conditions that would assure the operation is not hazardous, and determine how to detect these conditions independently of the PLC

c. Identify the electrical action that is caused by the PLC or the control hardware when the power is applied, removed and when errors are found. This information should be used for the designing of the expected and unexpected operations and to ensure the robust stability of the system. This information must NOT be used for safety purposes or limits.

d. Design the manual or electro-mechanical guide that properly inform the limits of the safety functioning of the device and blocks the operation independent of the control device implemented.

e. Provide appropriate information and states of the independent circuits of the PLC or control devices that are implemented in the process and any operator interfaces that must have the necessary information.

f. Detect any other safety-related requirements for the safe operation process.

4. Specify the Operator Stations: Based on the requirements of the functional specifications, create drawings of the operator stations. Include the following items [15]:

a. Overview showing the location of each operator station in relation to the process or machine.

b. Mechanical layout of the devices, such as display, switches, and lights, for the operator station.

c. Electrical drawings with the associated Input Output of the control device.

5. Create the configuration Drawings: Based on the functional specification requirements, the configuration and drawings must be implemented including the following elements [15]:

a. Overview showing the location of each I/O of the control device and its relation with the process or machine.

b. Mechanical layout of the control device and expansion modules, including extra equipment.

c. Electrical drawings for each control devise or extra equipment implemented, including the models numbers, general specifications, communication addresses and I/O addresses.

6. Create a list of Symbolic Names: The implementation of symbolic names instead of the address of the variables must be documented in a list that represent not only all the possible physical I/O signals but also the internal variables and memories used inside the program (local and global ones) [15].